Recently, in the production of plastic sheets including package materials, polyolefin-based substrate sheets (PO sheets) are increasingly used because of their being compatible with environment. However, in order for a given material to serve as a useful package material, it is often required that it is also amenable to secondary processing: particularly it must be easily printable and adhesive.
To meet the above requirement, that is, to improve the ease with which the PO sheet is amenable to secondary processing, a process is introduced to modify the surface condition of the PO sheet: the sheet is subjected to an oxidization treatment based on corona discharge, or a coating material comprising polyethylenimine and a resin emulsion is applied to the sheet.
However, the treatment based on corona discharge poses a problem: its effect fades over time.
Applying a coating material to a PO sheet is introduced with an intention to improve the printability of the sheet. However, because the PO sheet is inherently low in printability, the above treatment cannot bring about a notable effect as expected. In addition, the cationic activity inherent to polyethylenimine may impair the adhesive activity of a cyanoacrylate adhesive agent usually used in the secondary processing.
Under these situations, there is a need for a PO sheet, which is easily amenable to printing and bonding.
In the production of package materials, preparation of sheets consists of forming a hologram layer on a substrate sheet such as a PO sheet via metal vapor deposition, to enhance the ornamental effect of the sheet.
More specifically, forming a hologram layer on a substrate sheet consists of (1) preparing a metal film on a transfer material composed of PET or the like; (2) applying, under pressure, the transfer material to a substrate sheet via an adhesive layer by pressing a roller carrying the transfer material whose surface has a relief pattern against the substrate sheet; and (3) obtaining the substrate sheet having the metal film transferred and bonded thereupon with the film having the relief pattern printed.
The hologram layer will have its ornamental effect more enhanced, if both of the substrate sheet and the adhesive layer are made light-transmissive (transparent). The preferred adhesive agent for bonding a metal film to a PO sheet may include urethane-based agents or acrylic agents.
However, if a urethane-based adhesive agent is used in this process, although the adhesion is secured, the resulting product will have a limited ornamental effect, because the adhesive is not colorless nor transparent, On the other hand, if an acrylic adhesive agent is used in this process, the product will pose a different problem: the ornamental layer will be easily torn off because of its inadequate adhesion to the PO sheet, although the product will have a sufficient transparency. This is problematic in practice.
One object of the present invention is to provide a coating material useful for polyolefin-based sheets, which will improve the printability of the sheet, and not interfere with its adhesive activity, and also to provide display sheets, package sheets, transparent recording sheets, costume sheets, and emblem sheets based on the use of such coated polyolefin-based sheets.
Another object of the present invention is to provide a polyolefin-based sheet, which is suitably processed into a sheet having a metal-containing ornamental layer thereupon in which adhesion of the ornamental layer to the sheet is secured while the sheet assembly being kept transparent, and also to provide ornamental sheets based on the use of such polyolefin-based sheets.